Waste segregation presents thermal treatment opportunities

Abstract

Developments in national and international waste recycling and reuse programmes have led to the adoption of processes that segregate waste material into several streams. It is not viable to recycle some of these streams but some contain materials such as wood (biomass), contaminated paper (biomass), plastics and textiles that have a greater fuel energy content than the original waste. Furthermore, adding value through pelletisation can provide a convenient form for handling and transportation of this type of fuel. An objective of this investigation is to achieve electrical power generation efficiency greater than that of incinerator boiler/turbines through the generation of an intermediate gaseous fuel for use in a combined cycle. Studies of the pyrolysis process that produces char, oil and gas have demonstrated how the rate of heating influences the proportion of each. Complementary studies of the combustion and gasification characteristics of segregated waste materials, and char derived from them, in special pot ‘burners’ have shown the effects of composition, form, size, packing and airflow rate on burning and gasification rates. The results demonstrate the validity of a mathematical modelling code for the processes (Flic); when coupled with Fluent, this can then be used in the design and operation of domestic or industrial-scale plants. These investigations have led to the design of a small-scale gasifier that avoids the output of tar using pyrolysis in a bed of hot char. Such units are considered suitable for use with a Stirling engine for efficient combined heat and power (CHP) for individual buildings. An interesting feature of this system is that the overall CHP efficiency is independent of the gasifier efficiency.